[Discuss-gnuradio] Real sampling on the USRP

From:

Kunal Kandekar

Subject:

[Discuss-gnuradio] Real sampling on the USRP

Date:

Wed, 9 Nov 2005 16:04:31 -0500 (EST)

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Hi all,
I am working on a highly experimental project where I need to transmit and
receive noisy signals over the USRP, i.e. the noise is added digitally to
the signal at the transmitter itself.
Hence I would need to transmit and receive *only* real samples on the
USRP, without any filtering, interpolation/decimation or DUC/DDC.
Basically, I need to view the ambient electromagnetic energy level
*exactly* as it is at each moment of time:
antenna -> ADC -> USB -> S/W
S/W -> USB -> DAC -> antenna
Are there any gnu radio examples that show me how to do this?
I realize the bandwidth and range of IF (to adc_rate/2, i.e. 32MHz) will
be limited if I do this, but that is not a concern right now. I don't mind
working with lower frequencies. I suppose setting the interpolation and
decimation rates to 1 is the first step, and then fiddling with the mux
settings, but at what frequency should I set_tx/rx_freq(), if at all?
Furthermore, once I get the real samples, what should I do with them if I
want to, say, run FM tx/rx on the noisy signals?
Alternatively, if I try to transmit noisy signals with interpolation and
DUC, as normally used in the example programs, would the actually
transmitted signal still look like noise? With what characteristics? I can
understand quadrature sampling and modulation mathematically, but I cannot
bridge the gap between the math world and the physical world... especially
when I'm playing with noise!
This promises to be a most interesting project... I would really
appreciate any help I can get!
Thanks,
Kunal